It is well known in the field of LEDs that efficiency is generally improved as the LED die becomes smaller. For example, in the paper by Choi et al. entitled Mechanism of Enhanced Light Output Efficiency In InGaN-Based Microlight Emitting Diodes, Journal of Applied Physics, volume 93, number 10, 15 May 2003, Choi presents data showing the optical power density vs. current of an LED die increases about three fold when reducing the size of a micro-LED from 20 microns in diameter to 8 microns in diameter.
The present inventors had previously developed a technique for printing microscopic LEDs and connecting the LEDs in parallel. The LEDs are vertical LEDs with one electrode on top and the other electrode on the bottom. Each LED die has a diameter of about 30 microns and a thickness of about 7 microns.
FIG. 1 is a perspective view of one embodiment of the inventors' previously developed vertical LED 100, directly reproduced from U.S. Patent Application Publication 2013/0168658, incorporated herein by reference. As described below, the basic LED design has an inherent limitation that prevents it from being significantly smaller than 30 microns.
FIG. 2 is a top down view of the hexagonal LEDs 100 on a carrier wafer prior to singulation. The LEDs 100 are separated by trenches 102.
FIG. 3 is a simplified cross-sectional view of the LED 100.
An elongated metal top electrode 120B and its metal base 120A conduct current to the underlying p-type GaN layer 115. A bottom electrode (FIG. 3) 122 is formed on the bottom surface of a conductive substrate 105 (which may be n-GaN) to provide current to the overlying n-type GaN layer 110. At the junction of the p and n-type layers are light-generating quantum wells 112 (FIG. 3). The epitaxially grown LED semiconductor layers may be conventional. A metal via 130 (FIG. 1) may be used to bypass any low conductivity layers formed over the bottom electrode 122. The LED 100 is formed as a hexagon with sides 121.
The singulated LEDs 100 are designed to be suspended in an LED ink and printed over a substrate having a “bottom” conductor layer. A vast majority of the LED will have the same orientation. The bottom electrode 122 of the LED 100 contacts the bottom conductor layer. A dielectric layer is then deposited to insulate the bottom conductor layer and cover the sides of the LEDs yet expose the elongated top electrode 120B of the LEDs. A top conductor layer is then deposited to contact the top electrode 120B and connect the printed LEDs 100 in parallel. Either the top conductor layer or the bottom conductor layer, or both, are formed of a transparent conductor material so light exits one or both surfaces of the resulting light sheet.
The size of the LED 100 is limited by the smallest practical size of the top electrode 120B and its metal base 120A, which is roughly the relative size shown in FIG. 1. The top electrode 120A/B obscures and absorbs a substantial portion of the generated light. By making the LED die's diameter smaller, while the size of the top electrode 120A/B remains the same, there will be less light-generating area, and a higher percentage of the generated light will be obscured by the top electrode 120A/B, so efficiency drops.
Therefore, what is needed is a technique to form LED dies substantially smaller than 30 microns, to obtain an improved optical power output density vs. current, which are printable for manufacturing thin, flexible light sheets.